1. Field of the Invention
The present invention relates to a three-pin integrated synchronous rectifier and a flyback synchronous rectifying circuit that utilize a control pin to provide a control signal, whereby the control signal is used as a power bias voltage and a synchronizing signal, and a power transistor is built in a single package so that the connection pins are reduced. The structure is simplified so that the required area on the print circuit board is reduced and the cost of the power supply is decreased.
2. Description of the Related Art
The switching power supply traditionally is implemented by a diode rectifier. In order to meet the requirements, such as environmental protection, cooling and power efficiency, the diode rectifier gradually is replaced by the synchronous rectifier. However, the synchronous rectifying circuit of the prior art has a structure in which the control circuit and the power transistor are separated. In practical application, the less is the number of connection pins, the lower are the costs of the circuit. Therefore, the cost of the synchronous rectifier with three pins will be the lowest. One pin is controlled by power bias voltage and a synchronous pulse. The other two pins are the drain terminal and the source terminal of the power transistor. Due to the package limitation, the synchronous rectifying circuit of the prior art is implemented by an expensive synchronous rectifying control method that includes an eight pin synchronous rectifying controller, some discrete elements and an external power transistor.
FIG. 1 shows a synchronous rectifying circuit of the prior art. The synchronous rectifying circuit includes a DC power VIN, an input filtering capacitor C1, a turn-on resistor R1, a bias voltage power filtering capacitor C2, a primary pulse width modulation (PWM) controller 11, a transformer T1 having a primary main winding 12, an primary auxiliary winding 13 and a secondary output winding 20, a rectifying diode D1 providing a DC bias voltage power VCC, a primary side power transistor Q1 for controlling the power transmission of the transformer, a primary current detection resistor R2 for limiting the maximum output power, a secondary side power transistor Q1 for rectifying the secondary side, a synchronous rectifying control circuit 23 for controlling the turn-on and turn-off of the secondary side power transistor Q2, a feedback error-compensation amplifier 24, and a photo coupler 25. When the power is turned on, the DC power VIN charges the bias voltage power filtering capacitor C2 via the turn-on resistor R1. When the charged voltage reaches the turn-on voltage of the primary PWM controller 11, the primary PWM controller 11 outputs a turn-on signal to control the primary side power transistor Q1 to be operated so that the current flows into the primary main winding 12. When transformer T1 begins to operate, the bias voltage power is gradually provided by the rectifying diode D1 and the bias voltage power filtering capacitor C2, via the primary auxiliary winding 13. A primary side voltage feedback signal that is representative of the secondary side output voltage VO is transmitted to the primary side via the feedback error-compensation amplifier 24 and the photo coupler 25 and is inputted to the primary PWM controller 11. The synchronous rectifying control circuit 23 is supplied with power by output voltage VO, via the bias voltage power VDD and has two detection pins D, S respectively connected with a drain and a source of the secondary side power transistor Q2, and an output pin G connected with the gate of the secondary side power transistor Q2 to control the secondary side power transistor Q2 to be exactly turn-on or turn-off.
The synchronous rectifying circuit of the prior art needs to be packaged by at least four pin package. The low cost three pin industry standard packages, such as TO-220, DPAK, and TO-3P, are excluded. Furthermore, the power bias voltage VDD of the synchronous rectifying circuit of the prior art has to be a DC power. If the output voltage VO of the power supply is too high, the voltage needs to be reduced by a linear regulator. If the output voltage of the power supply is too low, a secondary auxiliary winding, rectifying and filtering elements and a linear regulator are required. Costs increase and the power efficiency decreases.